The present invention relates to a device for inspecting a fuel assembly in a pool of a nuclear plant equipped with a grapnel for displacing and suspending the assembly in the pool.
It also relates to an inspection method applying the device.
FIG. 1 schematically illustrates one of the racks 30 for storing assemblies 3 of fuel rods arranged in a storage pool 1 of a nuclear plant of the PWR (pressurized water reactor) type.
Each assembly 3 forms an elementary fuel unit and should be placed in a nuclear reactor in order to be able to produce energy. Each assembly 3 is formed with a series of supporting and separating grids, in which cylindrical rods containing pellets of enriched uranium oxide are positioned vertically. Each assembly generally has a square cross-section (with a side of about 200 mm) grouping, for PWR operated by EDF, 264 rods (geometry 17×17) and a height of the order of 4-5 m. End pieces 31 and 32 respectively form the top and the base of the assembly.
The racks 30, located at the bottom of the pool 1, include cells 34 with separating walls 33, so that no nuclear reaction occurs in the pool 1 when the assemblies 3 are placed in the racks 30. The so-called storage pool 1 forms a temporary storage lock for new assemblies 3 awaiting to be loaded into the nuclear reactor, and for spent assemblies awaiting removal (reprocessing, long period storage, etc.). The pool 1 is moreover filled with borated water 2, in order to avoid any nuclear reaction and any radiation towards the outside of the pool.
In order to displace the assemblies in the pool, for example for loading them into the reactor, or for unloading them from the reactor, or for examining them, a mobile grapnel 4, connected to a bridge 8 located above the pool, allows the assemblies 3 to be grasped by their top and to be taken out of the cells 34.
A lowerator 6 allows new assemblies to be put into the water. The lowerator 6 notably includes a basket 63, the length of which is generally at least equal to the length of an assembly 3, and in which an assembly is placed; this basket is mounted on a vertical rail 61 and actuated by a winch which allows it to be displaced along an axis Z parallel to a longitudinal axis of the assembly.
A transfer machine as for it allows transportation of the fuel assemblies from the reactor towards the pool 1 (and vice versa). A discharge ditch then allows transportation of the assemblies towards reprocessing plants.
In order to study or confirm the behavior under irradiation of the fuel assemblies, the possibility of inspecting the assemblies notably by metrology is desired.
FIG. 1 very schematically shows a known inspection device 10.
The device 10 is laid at the bottom of the pool or secured to the wall of the pool. It mainly includes a vertical enclosure 101 a little larger than the assembly, an enclosure open on one side in order to allow inspections. The fuel assembly is placed in this enclosure 101 by means of the bridge and of the grapnel 4. A video camera 102 mounted on the enclosure 101, may be displaced along the three axes (vertical 103, front/rear 105, left/right 104), in order to allow viewing of one face of the assembly. The camera is displaced by means of stepping motors 113, 114 and 115, the movement of which is indexed. A device placed at the bottom of the enclosure allows rotation of the assembly in order to allow successive examination of the four faces.
In order to carry out the inspection of an assembly, the assembly 3 is removed from a cell of the rack 30 by means of the bridge 8, and then the assembly 3 is placed in the enclosure 101. Measurements are carried out by displacing the camera 102 in the space in order to successively aim at both of the ends of each length to be measured; the indexation of the stepping motors has been calibrated beforehand on a standard with a reference length; the covered distance may therefore be inferred from the information given by the motors 113, 114 and 115.
EP 0 123 597 discloses an exemplary device, the operation principle of which is close to the known device described above.
The device of EP 0 123 597 includes means for supporting the assembly to be examined, means for examining the assembly, including a camera, and means for displacing the examination means.
Like for the device of FIG. 1, the fuel assembly should be mounted in the supporting means, prior to the measurements, for example by means of the grapnel already described.
The means for displacing the examination means include a mobile carriage, a column integral with the carriage, and a bracket vertically displaceable along the column. These specific displacement means are mounted in proximity to the supporting means.
Displacement counters are associated with the carriage and with the bracket in order to quantify the displacements of the camera.
Measurements require a preliminary step for calibrating the displacement counters. Calibration is carried out by observing with the camera a reference graduation attached on the supporting means in proximity to the assembly.
Once the displacement counters are calibrated, the measurement is conducted by successively aiming the camera at two ends of the length to be measured on the assembly: the displacement of the displacement means then provides the length to be measured.
Known devices of the state of the art however have drawbacks.
The known devices require the supply of specific additional pieces of equipment with respect to normal operation of the nuclear plant. These specific pieces of equipment are notably the enclosure 101 or the supporting means of EP 0 123 597, as well as the means for displacing the examination means. Such means may also be installed permanently, then requiring investment for each reactor.
Known devices are thus relatively bulky. Their presence in the pool may interfere with certain handling operations, such as removal of the used fuel for example.
The enclosure or the supporting means are moreover heavy. They have to be transported on the nuclear plant and installed in the pool for storing spent fuel. The transport and installation are costly operations which require significant time (one to two weeks).
Measurements take a long time, since the camera has to move in order to explore each of the components, the length of which is intended to be measured. The phase for calibrating the displacement counters also takes a long time.
The device includes a lot of elements and is not easy to decontaminate.
The cost of the device is further relatively high.
As the inspections are generally carried out during the renewal of the fuel, with a stopped reactor, the duration of the inspection may have a significant financial impact on the operating cost of the reactor.